The invention relates to a drafting unit for a spinning machine having several pairs of rollers which each contain a bottom roller and a pressure roller, and having an apron guide arranged between two pairs of rollers. An elastically tensioned bottom apron of this apron guide moves around a bottom roller and over a bottom apron bridge, the guiding surface of which is situated essentially in a common plane with the nip lines of the two pairs of rollers.
It is customary to mount bottom apron bridges in a stationary manner, particularly at roller stands receiving the bottom rollers. The required apron tensioning is generated in that a tensioning element is applied to the reversing end of the bottom apron. In a first known construction (Swiss Patent No. CH-PS 496 117), a tensioning roller is provided for the tensioning of the bottom apron, this tensioning roller being held on a pivoted spring-loaded holding device. In another construction (German Published Examined Application No. DE-AS 12 78 904), it is provided that a leaf-spring-type tensioning member is arranged at the holding device of the bottom apron bridge, this tensioning member being equipped with a spring arm which deflects and tensions the reversing end of the bottom apron.
An object of the invention is to construct a drafting unit of the initially mentioned type such that an improved tensioning and guiding of the bottom apron is obtained.
This object is achieved in that the bottom apron bridge is movably held at least approximately in the direction of its guiding surface and by means of a spring force is loaded in the direction away from the bottom roller around which the bottom apron is wound.
By means of this construction, it is achieved that the bottom apron is tensioned directly by means of the bottom apron bridge without the necessity of an additional tensioning element which additionally deflects the bottom apron. In particular, it is also ensured that the fiber-carrying end of the bottom apron is tensioned directly, i.e., by means of a force acting in its longitudinal direction so that the danger is reduced that folds, warpings, or the like are formed. In addition, it is possible to use a relatively short bottom apron. In addition, it is also possible to provide a relatively acute guiding at the end of the bottom apron bridge which faces the next pair of rollers. As a result, it is possible to continue to guide the bottom apron into the wedge-shaped gap so that the processed fiber material is guided longer.
In a further development of the invention, it is provided that movement of the bottom apron bridge in the direction of the spring force is limited by means of stop devices. As a result, the spring force tensioning the bottom apron can be precisely dimensioned by a limiting of movement and can be limited to a fixed value. The risk that the bottom apron is loaded excessively and therefore overstretched is eliminated.
In a further development of the invention, it is provided that the bottom apron bridge is mounted at a holder which can be swivelled around an axis which is parallel with respect to the bottom rollers and is loaded by means of a tension spring. The distance of the axis of the holder from the bottom apron bridge may be selected to be sufficiently large so that, in the required area, there is an approximately linear movement of the bottom apron bridge.
In a further development of the invention, it is provided that the bottom apron bridge, in the direction of its guiding surface, is form-lockingly inserted into the holder. As a result, the bottom apron bridge is securely fixed, on the one hand, while, on the other hand, it can still be detached and exchanged. In a constructively advantageous embodiment, it is provided that the holder is equipped with a U-shaped frame, in which the reversing end of the bottom apron moves and the legs of which are connected by means of the bottom apron bridge. In this embodiment, the legs of the U-shaped frame may at the same time be used as a lateral guide for the bottom apron.
In a further development of the invention, it is provided that the holder is equipped with a stop assigned to the bottom roller which follows. This stop will then define the exact position of the bottom apron bridge with respect to the nip line of the following pair of rollers. In this case, it is advantageous for the stop to be constructed as a component which is detachably and exchangeably arranged at the holder. As a result, it becomes possible, as necessary, to vary the position of the bottom apron and of the bottom apron bridge with respect to the nip line of the following pair of rollers by exchanging the holder for a holder with different dimensions and to adapt this position to the fiber material.
In a further development of the invention, a common holder is provided for the bottom apron bridge of two adjacent drafting units.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.